1. Field of the Invention
This invention relates to a parts feeding method and apparatus which feeds the parts such as a projection nut having projection for welding, a short thick pipe or a head bolt.
2. Prior Art
A conventional technology which is deemed nearest one to the present invention is disclosed in the Japanese patent publication No. 59-7549 and its gist is shown in FIGS. 12 and 13, wherein an inner shaft 2 is slidably inserted into a hollow outer shaft 1 and a magnet (permanent magnet) 4 for attracting a part (projection nut) 3 is fixed to an end of the inner shaft 2. An outer cylinder 5 which houses the outer shaft 1 is fixed to a stationary member 6 as shown in FIG. 12, and a pin 8 fixed to the outer shaft 1 is inserted into a guide groove 7 in an inner surface of the outer cylinder 5 in a direction of a stroke. A paired sheet metal part 9 is placed on a fixed electrode 10, and a guide pin 11 of the fixed electrode extends upward through a hole of the sheet metal part 9.
The end surface of both the shafts 1 and 2 are aligned to form the same one plane, on which a nut 3 is attracted. Remaining in this condition, both the shafts proceed to a predetermined position in one body, and on arrival at the position movement of the inner shaft 2 is stopped and only the outer shaft 1 proceeds further to separate the nut 3 from the magnet 4 as shown in FIG. 13, so that the magnetic attraction force applied to the nut 3 is substantially eliminated. By this actuation, the nut 3 separates from the end surface of the outer shaft 1 and jumps to the guide pin 11, to which a nut 3 hole is fitted in, whereby the feed is completed. A space transit distance L is shown in FIG. 13.
While a mechanism which stops advance of the shaft 2 is not shown in a figure, a limit pin fixed to the inner shaft 2 is projected from an elongated hole formed in the outer shaft so that when both the shafts 1 and 2 are advanced by a predetermined distance, whereby the advance of the inner shaft 2 is forced to stop as a result of the limit pin engaging with the stationary member, and only the outer shaft 1 advances to reduce the magnet 4's attraction force on the nut, the nut inertially falling toward the guide pin 11.
In the conventional technology mentioned above, the magnetic attraction force on the nut 3 is substantially eliminated during a transient period in which the magnet stops and the outer shaft is further advancing, therefore the following problems arise. That is, the first problem is that since the attraction force on the nut is gradually decreased while only the outer shaft is advancing, when some external force or vibration acts on the nut before it reaches a predetermined position, the nut can sometimes fall to cause a feed failure. The second problem is that a feed mechanism depends on substantial elimination of the attraction force on the nut, but timing at which the nut separates from the outer shaft is not constant owing to a variation of nut weight or viscosity of oil on the nut. Therefore, space transit distances of the nuts vary from a short to a long one, thus causing a trouble with feed operation stability. Thus, owing to variation in the space transit distance, this distance is required to be set to a longer one in favor of the longest, and this makes the space transit distance long, causing feed failures.
Furthermore, in the above-mentioned conventional technology, the attraction force of the magnet 4 fails to act sufficiently on the nut 3 because of a nut hole, and mounting the magnet 4 on the inner shaft 2 needs to limit a magnet size. In addition, since the nut 3 contacts only end surfaces of both the shafts 1 and 2, some external force or vibration can displace the nut 3 to impair fitness between the nut hole and the guide pin 11, causing the nut to be flipped by the guide pin 11.
Searching further for causes of the above problems, they originate in the structures for magnet installation. That is, in the conventional technology, the magnet is fixed to a tip of the inner shaft as shown in the figures, so that it is impossible to enhance the attraction force on a part if a hole is formed in the part as in a nut, thus a fall of the nut is caused by the external force or vibration as mentioned above.
A still more important problem is that insufficient magnetic attraction force obstructs providing the feed rod with a higher speed advance, thus bringing a great disadvantage in time-shortening for parts feeding.